1. Field
The present invention generally relates to a technique of reducing interference occurring in signal lines when connecting electronic devices, each of which is electromagnetically shielded, to each other.
2. Description of the Related Art
A notebook computer (hereinafter, referred to as a ‘notebook PC’) is very portability since the notebook PC is small and light, but the notebook PC has a slightly limited functionality compared with a desktop computer. To expand the functionality of a notebook PC when using the notebook PC in an office or a house, a function expansion device called a docking station is adopted. The function expansion device may be provided with storage devices, such as a CD-ROM drive and a hard disk drive, connecting terminals, such as a serial port, a parallel port, and a USB, expansion slots of various kinds of buses, and the like. In addition, by connecting a notebook PC to a function expansion device with a connector, desktop computer functions can be enjoyed and the complications of connecting to a network, a printer, and the like can be avoided. A function expansion device including only connecting terminals, such as a serial port, a parallel port, a USB port, an external display output connector, and a connector for a printer, is commonly referred to as a port replicator.
Since the notebook PC and the function expansion device accommodate electronic devices employing high-frequency signals, electromagnetic waves are emitted from the notebook PC and the function expansion device. In addition, the notebook PC and the function expansion device may be easily affected by electromagnetic waves introduced from the outside. Therefore, in the notebook PC and the function expansion device, electromagnetic shielding is typically used to prevent electromagnetic interference (EMI). Hereinafter, the electromagnetic shielding is referred to as an EMI shield. The EMI shield covers an electronic device with a thin plate formed of a conductive material, such as aluminum or copper, that reflects or absorbs electromagnetic waves emitted from the inside and electromagnetic waves introduced from the outside, so that that the electromagnetic waves emitted from the inside and the electromagnetic waves introduced from the outside do not pass through the EMI shield.
A circuit in the notebook PC and the function expansion device is typically configured to include a signal line through which a high-frequency pulse signal flows and a signal earth line which serves to apply a reference potential to the signal line. Since the EMI shield applies a common reference potential to various electronic devices of the notebook PC and the function expansion device, and the signal earth line of each electronic device is connected to the EMI shield. The notebook PC and the expansion device are provided with interface connectors used for connection therebetween, and each signal line and each signal earth line are connected to the corresponding interface connector.
In case where a signal earth line is connected to a corresponding EMI shield, an EMI shield of a notebook PC and an EMI shield of a function expansion device are electrically connected to each other through the signal earth lines when the notebook PC and the function expansion device are connected to each other with interface connectors. However, it is difficult to make the EMI shields have the same electric potential during an operation of the notebook PC by only connecting the signal earth lines to each other.
The resistance of a notebook PC to noise tends to decrease as an operating frequency of the notebook PC increases and an operating voltage of the notebook PC decreases. In addition, electric resistance tends to increase as an EMI shield becomes thinner for reduction in weight. As a result, a connecting structure of a notebook PC and/or a function expansion device may function as a lightning rod, causing the notebook PC may malfunction due to aerial discharge of electrostatic charge when hot docking the notebook PC.
FIG. 5 is a schematic block diagram illustrating a malfunction when a notebook PC is docked with a docking station function expansion device. A notebook PC 10 includes an EMI shield 113, and a docking station 50 includes an EMI shield 143. A mother board 115 and an electronic device 117 are accommodated inside the EMI shield 113, and an electronic device 145 is accommodated inside the EMI shield 143. Circuit elements 125 and 127 are mounted on the mother board 115, a circuit element 129 is mounted on the electronic device 117, and a circuit element 155 is mounted in the electronic device 145.
A signal line 121 and a signal earth line 123 of the circuit elements 125 and 127 are connected to an interface connector (hereinafter, referred to as a ‘connector’) 15. A signal line and a signal earth line of the circuit element 129 are connected to the signal line 121 and the signal earth line 123 of the mother board. The signal earth line of the circuit element 129 is also connected to the EMI shield 113. A signal line 149 and a signal earth line 151 of the circuit element 155 are connected to a connector 55. The signal earth line 123 is connected to the EMI shield 113, and the signal earth line 151 is connected to the EMI shields 141. When a housing (not shown in FIG. 5) provided outside each of the EMI shields 113 and 143 is a conductor, the housing and each of the EMI shields 113 and 143 are electrically connected to each other. EMI connecting protrusions 59a and 59b and lightning protrusions 157a and 157b are provided in the EMI shield 143.
When the connector 15 and the connector 55 are brought closer to each other in order to hot dock the notebook PC 10, on which electrostatic charge is carried, with the docking station 50, the electrostatic charge is discharged through a space between the lightning protrusions 157a and 157b and the EMI shield 113. The discharge of the electrostatic charge is referred to herein as electrostatic discharge (ESD). When the ESD is through the air, a rapid movement of electric charges occurs. As a result, a convection current flows in the air and a conduction current flows in the EMI shield 113 of the notebook PC 10. Since the conduction current is an impulse-shaped large current, a harmonic component is included. Accordingly, an inductive reactance of the EMI shield 113 also acts as large impedance. As a result, a local fluctuation in electric potential occurs in the EMI shield 113 due to impedances 131 and 133 each having resistance and inductive reactance.
In addition, due to electrostatic coupling or electromagnetic coupling between the EMI shield 113 and the signal line 121, noise is introduced into the signal line 121 so that a reference potential of the circuit element 129 is changed. Moreover, since harmonic components are also included in a convection current, electromagnetic wave noise is generated also from an aerial discharge portion. As a result, the notebook PC 10 may malfunction. Furthermore, depending on the position of the notebook PC when connecting the notebook PC 10 and the docking station 50, the EMI connecting protrusions 59a and 59b may be brought closer to the EMI shield 113 earlier than the lightning protrusions 157a and 157b such that the electrostatic charge between the EMI connecting protrusions 59a and 59b is discharged through the air, causing a malfunction while hot docking the notebook PC 10.
A user who uses a notebook PC in an office may perform so-called hot docking, that is, may connect the notebook PC to a function expansion device when power is not turned off, such as when the user comes back to the desk after using the notebook PC in the meeting. At this time, the notebook PC is electrically charged with static electricity from the user holding the notebook PC, and accordingly, an electrostatic charge is generated. When the notebook PC on which electrostatic charge is accumulated is hot docked to the function expansion device, ESD occurs between interface connectors when the notebook PC and the function expansion device are brought close to each other. As a result, a discharge current flows through a signal earth line or a signal line, which may cause the notebook PC to malfunction.